Aesop's Fables, translated by Laura Gibbs (2002)

A story about an eagle and a snake, exhorting us to be the first to
grant a favour.
A snake and an eagle were grappling with one another as they fought. The
snake had tightened his hold on the eagle when a farmer saw them and freed
the eagle from the grip of the snake. The snake was angry about what had
happened, so he went and poisoned the man's drinking water. But just as
the unsuspecting farmer was about to take a drink, the eagle flew down
and snatched the cup out of his hands. The man who treats others well is rewarded by gratitude.

Note: There is an elaborate version of this story in Aelian, Characteristics
of Animals 17.37.

Source:
Aesop's Fables. A new translation by Laura
Gibbs.
Oxford University Press (World's Classics): Oxford, 2002.
NOTE: New
cover, with new ISBN, published in 2008; contents of book unchanged.